1. Field of the Invention
This invention relates to a video signal combining apparatus and method for combining three or more video signals.
2. Description of the Prior Art
As a video signal combining apparatus which obtains an apparently three-dimensional video signal by combining a plurality of two-dimensional video signals, it is known to provide a DME (digital multieffector).
FIG. 2 of the accompanying drawings shows the fundamental structure of a conventional 3-channel DME combiner for obtaining a three-dimensional video image by combining three video signals.
This 3-channel DME combiner is constituted by having two, a first and second, 2-channel combiners 1 and 2 so-called cascade-connected, i .e . first having two video signals DME.sub.1 and DME.sub.2 combined by the first 2-channel combiner 1 and then having this combined video signal DME.sub.12 and the remaining video signal DME.sub.3 combined by the second 2-channel combiner 2, thereby obtaining a desired combined video signal DME.sub.123.
In such a structure, the video signals input to the first and second 2-channel combiners 1 and 2 comprise a video signal V, a key signal K, and a depth signal Z. By appropriately combining the key signal K and the depth signal Z by a desired value, it is possible to make the edge of the combined video image and an intersecting region non jagged and smooth.
Here, the basic edge processing in the combining operation of the 2-channel combiner will be explained using FIG. 3 of the accompanying drawings and taking as an example the case where two planes A and B intersect. Note that, in the figure, BG denotes a background; a an edge of a region in which a plane A exists on a plane B; b an edge of a region in which the plane B exists on the plane A; c an edge of a region in which the plane A exists on the background; and d an intersecting region between the plane A and the plane B.
Edge a; As shown in the following equation (1), video signals V.sub.A and V.sub.B are mixed using a key signal K.sub.A of the A input: EQU K.sub.A.V.sub.1 +(1-K.sub.A).V.sub.B ( 1)
(where, 0.ltoreq.K.sub.A .ltoreq.1)
Edge b: As shown in the Following equation (2), video signals V.sub.A and V.sub.B are mixed using a key signal K.sub.B of the B input: EQU K.sub.B.V.sub.B +(1-K.sub.B).V.sub.A (2)
(where, 0.ltoreq.K.sub.B .ltoreq.1)
Edge c: As shown in the following equation (3), a picture F and a background BG are mixed using a combined key signal K.sub.BG of the key signals K.sub.A and K.sub.B : EQU K.sub.BG.V.sub.F +(1-K.sub.BG).V.sub.BG (3) EQU K.sub.BG =1-(1-K.sub.A)(1-K.sub.B)
Intersecting region d: A priority signal Z.sub.AB, which is one type of key signal, is produced using the depth signals Z.sub.A and Z.sub.B, and video signals V.sub.A and V.sub.B are mixed as shown in the following equation (4): EQU Z.sub.AB.V.sub.A +(1-Z.sub.AB).V.sub.B (4)
The 2-channel combiner obtains a combined video image having a smooth edge and intersecting region by performing the above processing.
However, the conventional B-channel DME combiner comprising two cascade-connected 2-channel combiners having the above-described Function suffers From the problem that a smooth edge cannot be obtained on the basis of the order of combination of the video signals corresponding to the images to be combined without account being taken of the depth relationships of the images. This results in unnatural edges of a step-like form.
A more detailed description will be given below regarding this problem.
Note that, here, For simplification of the explanation, a case where three plane images of A, B, and C are superimposed in an order of A, B, and C from the top, as shown in FIG. 4 of the accompanying drawings, will be explained using FIG. 5 and FIG. 6 of the accompanying drawings.
As shown in FIG. 5(a), in a case where after the video signal DME.sub.A of the image A and the video signal DME.sub.B of the image B are combined, the combined video signal DME.sub.AB and the video signal DME.sub.C of the image C are combined, that is, where the signals are combined in an order of depth, in the combined video image AB of A and B, A and B are mixed by the key signal K, so that the edge of A is smooth.
Then, even in a case where the combined image AB and the image C are combined, B and C are mixed by the key signal K, so that the edge of B becomes smooth.
Contrary to this, as shown in FIG. 5(b), in a case where after the video signal DME.sub.A of the image A and the video signal DME.sub.C of the C are combined, the combined video signal DME.sub.AC and the video signal DME.sub.B of the image B positioned in the middle of the two images A and C are combined, the edge of B becomes smooth since B and C are mixed by the key signal K, but the edge of A exhibits jaggedness since an appropriate key signal does not exist.
Specifically, the image A and the image C are combined in advance, so the picture of C which should not be seen in the edge part of A appears as shown in FIG. 6(a).
On way to avoid the appearance of picture C would be not to combine the images A and C, but in this case, as shown in FIG. 6(b), the edge exhibits jaggedness.
This problem can be avoided if the combination is performed from the highest (or lowest) order in the order or arrangement (depth), but unlike in the case of a switcher, in the case of a DME combiner, generally the order of arrangement of pictures varies for each pixel and is not uniform at all positions in the picture plane, and therefore the above-described problem cannot be simply solved.
Also, in a case where the image A is semi-transparent and the image B is opaque, a problem similar to that mentioned above occurs. Namely, when the image A and the image C are combined in advance, A and C are mixed, and the picture of C which should be invisible since B is opaque is sometimes seen.
An example of the type of video signal combining circuit described above is shown in U.S. Pat. No. 4,851,912 (The Grass Valley Group Inc.)